1. Field of the Invention
This invention relates to an electric power steering device.
2. Description of the Related Art
Many of the motor control devices that are used in electric power steering devices (EPS) and the like are provided with abnormality detecting means for detecting an abnormality when an electrification failure occurs in any of a three-phase motor (any of U, V and W phases) due to disconnection of a power supply line or a contact failure of a drive circuit. Upon detection of the abnormality, these motor control devices stop the motor control promptly to attain fail safe.
However, the steering characteristics of the EPS change significantly due to the discontinuation of the motor control. In other words, a greater steering force is required for the driver to perform accurate steering operation. There is a motor control device in which even when a phase with an electrification failure is detected, motor control is continued with the other two electrification phases (see Japanese Patent Application Publication No. 2003-26020 (JP-A-2003-26020), for example). With this device, an assist power can be continuously applied to the steering system to avoid an increase of burden that is imposed on a driver when attaining fail safe.
However, in the case described above in which an electrification failure occurs in a phase and therefore the motor control is continued with the other two electrification phases, deterioration of steering feeling occurs inevitably due to the occurrence of a torque ripple, in a configuration where a sinusoidal waveform current is applied to each electrification phase, as shown in FIG. 20 (in the illustrated example, the U phase is abnormal, and V and W phases are electrified).
Specifically, as shown in the d/q coordinate system of FIG. 21 which shows how the currents change when the two phases of the motor are driven, the actual q-axis current value changes sinusoidaly, in spite of the constant q-axis current command value, which is a control target value of a motor torque. In other words, because a motor current corresponding to a requested torque is not generated, the motor is continuously driven without delivering its true output performance, causing a significant change in the assist power of the motor. This problem leaves room for improvement.